1. Field of the Invention
This invention relates to linear hollow charge devices and in particular but not exclusively to linear cutting charge devices.
2. Discussion of Prior Art
Hollow charges, also commonly referred to as shaped charges, are known comprising a mass of explosive having a concavity in one of its surfaces, the concavity being lined with a ductile metal liner. Detonation of the charge violently comprises the ductile liner converting it into an outwardly-projected elongated jet of metal, the shape of which is largely dependent upon the shape of the concavity. The jet has powerful penetrating properties which are utilised by detonating the charge with its concavity adjacent and facing a surface to be penetrated i.e. the work surface. The degree of surface penetration is dependent upon the distance, known as the stand-off distance, between the charge and the work surface. The optimum value of this distance is normally determined by experiment.
Most known types of hollow charge device are axisymmetric in which the charge and its shaped cavity have circular symmetry about a fore-and-aft axis. The cavity is typically formed as reentrant hollow cone, and the correspondingly-axisymmetric jet penetrator formed from the collapsed liner is projected along the line of the axis. However, some types of hollow charge device are non-axisymmetric and are designed to produce non-axisymmetric jet penetrators.
One particular example of a non-axisymmetric hollow charge is disclosed in UK Patent Application No. GB2176878A, which discloses a linear cutting charge comprising an extruded bar, formed from a composite of explosive material and a first plastic material, having a longitudinal planisymmetrical cavity along its length in the form of a "V"-shaped groove which is lined with an extruded liner of a metal/plastic composite. The bar is enclosed in a non-metallic casing which includes a casing portion providing a barrier between the cavity and the working surface of a thickness equal to the optimum standoff distance for the cutting charge. Detonation of the bar produces a jet along the length of the groove which can be utilised for cutting purposes.
The detonation of a hollow charge is usually initiated by a single detonator located at the end of the charge remote from the concavity. In an axisymmetric hollow charge device, the detonator is usually located axially. However, since a single detonator will produce a generally spherical wavefront which propagates through the charge to strike the concave liner at a rather oblique angle, only a relatively small proportion of the energy in this detonation wavefront is transmitted to the liner so that a jet penetrator of relatively low penetration efficiency is formed and much of the available energy from the detonated charge is wasted. In a linear hollow charge device, this problem of low penetration efficiency is further complicated by the non-axisymmetrical shape of the charge. The spherical detonation wavefront generated from each of one or more detonators situated on or along the charge will strike the liner at different angles along the length of the charge. This leads to undesirable variations in the cutting efficiency of the jet formed along the length of the groove.
In a yet further design of a linear hollow charge device disclosed in British Patent Specification GB2138111A, the initiation of the device is controlled by providing the cutting charge in two parts consisting of an initiating charge and a main charge. The initiating charge has a cavity in one end lined with an inert transfer plate. The main charge, with its lined groove facing away from the initiating charge, is located within this cavity to leave a gap between the facing surfaces of the main charge and transfer plate. Initiation of the initiating charge projects the transfer plate across the gap to initiate the main charge. Initiation of the main charge is affected by such factors as the contours of the facing surface of the main charge and transfer plate, and the phase velocity V.sub.Ph at which the plate strikes the main charge across its surface. Single-point initiation of the initiating charge as disclosed in GB2138111A creates out-of-phase projection, both along and across the linear device, of the transfer plate towards the main charge, creating a complex pattern of initiation on the facing surface of the main charge. This in turn leads to poor penetrator performance.